The autoresonant accelerator is a combination of the traveling-wave and collective accelerator concepts. The medium in which the wave travels is a relativistic electron beam immersed in a dc magnetic field. This traveling wave has the property that its phase velocity increases with decreasing magnetic field. In order to accelerate ions which are in resonance with the wave, the phase velocity of the wave must be adiabatically increased from a low phase velocity at the input end of the accelerator where the ions are loaded onto the wave to a higher phase velocity at the output end of the accelerator where the ion beam must be extracted without loss of beam quality due to scattering.
For all applications of the very high energy ion beams which require propagation through the atmosphere, it is necessary to extract the beam with as little loss of beam quality as practical. Here beam quality is determined by the ratio of energy due to transverse motion (that is temperature as measured in a frame moving with the velocity of the beam) to the energy due to motion in the beams direction as measured in the labs frame. The pressure in the accelerator tube and the high voltage diode of the electron beam generator is of the order of 10.sup.-4 torr or less. Some means must be used to prevent leakage of the ambient atmosphere into the accelerator tube where the ion beam is extracted. The simplest means of supporting the pressure difference would be with a solid thin film window. However, available windows absorb energy from the beam and reduce the beam quality due to scattering. At these high energy fluxes (approximately 5 .times. 10.sup.12 watts for 270 nanoseconds) the heating caused by the absorption of even a very small fraction of the beam energy not only scatters the beam particles but it also weakens the window causing it to rupture so that the accelerator cannot be operated again until the window has been replaced and the vacuum in the accelerator reestablished. Differential pumping using vacuum pumps might be possible but pumping speeds like 3.5 .times. 10.sup.8 liters/sec at 10.sup.-4 torr would be required for an aperture of only 1 cm.sup.2, which is smaller than the apertures required here.